According to the prior art the vapour by-products from the esterification, prepolycondensation and polycondensation stages consisting of aldehydes, water, ethylene glycol, diethylene glycol and oligomers are treated in various ways.
The vapours from the esterification stages are condensed and cooled from 260.degree.-270.degree. C. to 90.degree.-95.degree. C. in surface condensers. The condenser surfaces are washed with ethylene glycol to prevent the solids collecting. Frequently, the condensers are shut down and the oligomer deposits are melted and removed at high temperature. The necessary excess ethylene glycol in the esterification stages is maintained by feeding a cold EG/TPA paste with a higher mole ratio (1.5 to 1.8).
The ethylene glycol is fed to the esterification stages at 25.degree.-30.degree. C. where it is heated and vapourized at 260.degree.-270.degree. C.; the vapours are then condensed and cooled. Finally the ethylene glycol is recovered from the condensate through successive rectification. This process is very inefficient, has high ethylene glycol and energy consumption, and is carried out in complicated equipment (condensers with stand-by pumps, ethylene glycol recovery plant, hold tanks, etc.).
Because of high heat loads in the first esterification stages the heat exchanger surface has a temperature higher than 275.degree. C. which increases the diethylene glycol formation rate. To prevent this phenomena diisopropylamine (DIPA) is added, an organic base which reduces the acid-catalysed reaction rate of ethylene glycol to form diethylene glycol. Unfortunately DIPA is an undesirable component which changes the colour of the polyester end product and also has a high toxicity.
Other processes using an in-line distillation column have been developed to avoid the above disadvantages. The vapours from the esterification stages are fed into the column bottom that has only a rectifying zone. The overhead vapours are withdrawn from the top of the column and condensed. A portion of the condensate is returned to the top of the column as external reflux and the rest is sent to waste. The bottom product consisting of ethylene glycol contaminated with diethylene glycol and oligomers is returned to the esterification stages.
An enriching column has the function of purifying the lower boiling constituent, water. However, this device is incapable of producing the very pure ethylene glycol bottoms product necessary for the esterification process because even if the column had an infinite number of stages, the bottoms composition would contain 5 wt % water which reduces the rate of the esterification reaction. In order to reduce the water concentration of bottom product to approximately 0.5 wt % the column is operated to produce distillate with 1.4 to 1.6 wt % ethylene glycol. Consequently the high concentration of ethylene glycol in the distillate (1.4-1.6 wt %) produces high losses of ethylene glycol and an unjustified high operating cost for waste water treatment.
Moreover, the ethylene glycol bottom product returning to the esterification process is contaminated with diethylene glycol and oligomers such as diglycol acid mono glycol ester. By recycling this impure ethylene glycol the concentration of diethylene glycol in the esterified product increases by an average of 30 to 40 percent. The presence of significant amounts of diethylene glycol in the esterified product is particularly intolerable for the quality of the end product.
Another disadvantage of all prior processes is that the recovery of ethylene glycol from outgoing vapours of pre- and polycondensation stages is carried out in a complicated distillation plant. The yield of recovered ethylene glycol is only 85-90 percent.